The prostate-specific membrane antigen (PSMA) is a type II integral membrane protein that is abundantly expressed in prostate cancer and the endothelium of neovasculature in most solid tumors. PSMA is increasingly recognized as an important target for cancer imaging and therapy.
Over the past decade, a broad range of radiolabeled compounds have been synthetized for imaging and therapy of prostate and many other types of cancer and tumor neovasculature in combination with noninvasive imaging techniques, such as magnetic resonance imaging (MRI), which achieves high spatial and temporal resolution. The imaging probes for magnetic resonance imaging (MRI) are termed “contrast agents,” since they enhance the water proton-based contrast between the imaging target and the surrounding tissue. Detection with MRI relies on contrast in the MRI signal between the tissue of interest and its surrounding tissue.
Recently, a new type of MRI contrast that relies on direct chemical exchange of protons with bulk water has been developed and is referred to as chemical exchange saturation transfer (CEST) MRI. CEST MRI is a technique in which concentration marker molecules are labeled by either saturating or labeling their exchangeable proton spins by radio-frequency (RF) irradiation. If such saturation or labeling can be achieved rapidly, i.e., before the spin exchanges, exchange of such labeled spins with water leads to transfer of the magnetization, allowing indirect detection of the solute via the water resonance through a change in signal intensity in MRI.
A variety of organic molecules possessing protons that exchange rapidly with the surrounding water protons have been suggested as new contrast agents. These exchangeable protons can be “magnetically tagged” using a radiofrequency saturation pulse applied at their resonance frequency. The tagged protons exchange with the protons of surrounding water molecules and consequently reduce the MRI signal. This effect in and of itself would not be visible at the low concentrations of solute, but the exchanged protons are replaced with fresh, unsaturated protons and the same saturation process is repeated. Over time (e.g., several seconds) this repetition results in signal amplification and very low concentrations of agents can be detected. Hence, these agents are termed CEST contrast agents.
Each CEST contrast agent can have a different saturation frequency, which depends on the chemical shift of the exchangeable spin. The magnitude of proton transfer enhancement (PTE) due to this effect, and the resulting signal reduction from equilibrium (S0) to saturated (Ssat), are given by:
                    PTE        =                                                            NM                w                            ⁢              α              ⁢                                                          ⁢                              k                ex                                                                                      (                                      1                    -                                          x                      CA                                                        )                                ⁢                                  R                                      1                    ⁢                    wat                                                              +                                                x                  CA                                ⁢                                  k                  ex                                                              ·                      {                          1              -                              e                                                      -                                          [                                                                                                    (                                                          1                              -                                                              x                                CA                                                                                      )                                                    ⁢                                                      R                                                          1                              ⁢                              wat                                                                                                      +                                                                              x                            CA                                                    ⁢                                                      k                            ex                                                                                              ]                                                        ⁢                                      t                    sat                                                                        }                                              [                  Eq          .                                          ⁢          1                ]                        and                                                                (                      1            -                                          S                sat                            /                              S                0                                              )                =                                            PTE              ·                              [                CA                ]                                                    2              ·                              [                                                      H                    2                                    ⁢                  O                                ]                                              .                                    [                  Eq          .                                          ⁢          2                ]            wherein “CA” is the contrast agent containing multiple exchangeable protons, xCA is its fractional exchangeable proton concentration, α is the saturation efficiency, k is the pseudo first-order rate constant, N is the number of exchangeable protons per molecular weight unit, and Mw is the molecular weight of the CA. The exponential term describes the effect of back exchange and water longitudinal relaxation (R1wat=1/T1wat) on the transfer during the RF saturation period (tsat). This effect will be larger when spins exchange faster, but, under such conditions, saturation must occur faster, as well, which increases the radio-frequency power needed. In addition, the resonance of the particular spins must be well separated from the bulk in the NMR spectrum, which requires a slow exchange on the NMR time scale. This condition means that the frequency difference of the exchangeable spins with the bulk is much larger than the exchange rate (Δω>k).
Thus, the CEST technology becomes more applicable at higher magnetic fields or when using paramagnetic shift agents. Any molecule that exhibits a significant PTE effect can be classified as a CEST (contrast) agent. The concept of these agents as MR contrast agents is somewhat similar to the chemical amplification of colorimetric labels in in situ gene expression assays. For instance, CEST agents can be detected by monitoring the water intensity as a function of the saturation frequency, leading to a so-called Z-spectrum. In such spectra, the saturation effect of the contrast agent on the water resonance is displayed as a function of irradiation frequency.
Since the first report of CEST contrast in 2000, CEST MR imaging has become a widely used MRI contrast mechanism, and CEST contrast is generated by the dynamic exchange process between an exchangeable proton of a biomarker of interest and the surrounding water protons. To detect the biomarkers, the magnetization of some of their exchangeable protons is nullified by applying a selective radiofrequency saturation pulse at the specific resonance frequency (chemical shift) of the target protons. Due to exchange of the “saturated” agent protons with surrounding water protons, the net water signal is reduced, thus enhancing the MRI contrast.
CEST contrast agents have many advantages, such as lower toxicity due to the absence of lanthanide metals, ease of modification, and clearance through breakdown during natural biochemical processes. However, currently reported organic CEST agents suffer from sensitivity drawbacks, however, especially due to a small chemical shift difference between exchangeable proton and water. There are many challenges with respect to detecting CEST contrast agents, such as low spatial/temporal resolution, artifacts and low contrast-noise-ratio, and difficulties separating CEST contrast from other sources of water signal loss. Therefore, there remains a need for the design and development of MRI contrast agents that offer improved sensitivity and contrast effects in producing MR images.